Polaxial Bone Anchors With Increased Angulation

ABSTRACT

A polyaxial bone anchor has a locking element shaped and configured to allow an anchoring member (e.g., a screw or hook) to polyaxially rotate at large angles about a central axis of the bone anchor before compression locking the anchoring member within an anchor head.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/187,947, filed Feb. 24, 2014, now allowed, which is a continuation ofU.S. patent application Ser. No. 13/688,600, filed Nov. 29, 2012, nowU.S. Pat. No. 9,504,498, issued Nov. 29, 2016. U.S. patent applicationSer. No. 13/688,600 is a continuation of U.S. patent application Ser.No. 13/329,755, filed Dec. 19, 2011, now U.S. Pat. No. 8,679,162, issuedMar. 25, 2014. U.S. patent application Ser. No. 13/329,755 is acontinuation of U.S. patent application Ser. No. 11/603,428, filed Nov.21, 2006, now U.S. Pat. No. 8,100,946, issued Jan. 24, 2012. U.S. Pat.No. 8,100,946, as well as each application listed in this paragraph,claims the benefit of U.S. Provisional Application No. 60/739,100, filedNov. 21, 2005.

TECHNICAL FIELD

This invention relates to bone fixation devices and related methods offixation. More particularly, this invention relates to polyaxial boneanchors, such as pedicle screws and hooks, having increased angulationfor use in, for example, the posterior fixation of the spine.

BACKGROUND

Polyaxial bone anchors and methods of use in treating spinal disordersare known. Typical methods involve anchoring at least two screws orhooks into the vertebrae, and fixing the screws or hooks along a spinalrod to position or immobilize the vertebrae with respect to one another.The screws or hooks commonly have anchor heads with U-shaped channels inwhich the spinal rod is inserted and subsequently clamped by a fastener,such as, for example, a threaded nut, set screw or locking cap. Thesemethods commonly involve multiple screws or hooks and multiple spinalrods. The spinal rod(s) may be shaped to maintain the vertebrae in adesired orientation so as to correct the spinal disorder at hand (e.g.,to straighten a spine having abnormal curvature). Additionally oralternatively, the screws or hooks may be spaced along the rods(s) tocompress or distract adjacent vertebrae.

Surgeons may encounter difficulty with spinal fixation and stabilizationmethods because of difficulty aligning the spinal rod(s) with theU-shaped channels in the anchor heads of the screws or hooks. Forexample, the anchor heads are often out of alignment with one anotherbecause of the curvature of the spine or the size and shape of eachvertebrae. To facilitate easier insertion of the spinal rods into theU-shaped channels, and to provide additional flexibility in thepositioning of the spinal rods and the screws and hooks, bone anchorshave been developed where the anchor member (e.g., screw or hook) andanchor head can initially pivot or rotate with respect to each other.These bone anchors are sometimes referred to as polyaxial bone anchorsand the pivot or rotation of the anchor member is referred to asangulation.

A disadvantage of many polyaxial bone anchors is the degree to which theanchor head and member can angulate. Typical polyaxial bone anchors haveanchor members that can rotate up to about 30° from a central axisextending down through the anchor head. It may be advantageous toprovide polyaxial bone anchors with increased angulation.

SUMMARY OF THE INVENTION

The invention is directed to polyaxial bone anchors and methods of usefor attaching a rod, such as a support or spinal rod, to a bone, such asa vertebra. The bone anchor may include a hollow generally cylindricalhousing or head (referred to hereinafter as an anchor head), an optionalhollow generally cylindrical internal sleeve, an internal lockingelement, a pedicle screw for other type of anchor member, such as, forexample, a hook or other similar structure), and preferably a lockingcap with set screw (alternatively, other types of fasteners andfastening arrangements, such as, for example, a threaded nut or lockingsleeve mounted on or over the top portion of the head, are also withinthe scope of the invention). The anchor head and internal sleeve mayhave a U-shaped channel for receiving a support/spinal rod (referred tohereinafter as a spinal rod or rod). The locking element preferably issized and shaped to snap on to the head of the pedicle screw. And thelocking cap and set screw may close the top opening of the U-shapedchannel after a rod has been placed therein and, in combination with thelocking element, lock or clamp the respective positions of the pediclescrew and rod.

The anchor head, the internal sleeve, and primarily the locking elementhave features that allow the locking element to rotate or pivot withinthe anchor head. This in turn allows the pedicle screw to rotate orpivot around and away from the central axis of the anchor head at largeangles. The pedicle screw or hook may be locked with respect to theanchor head at these large angles. The angulation is preferably as muchas about 50° in every direction from the central axis. Thisadvantageously provides greater flexibility to the surgeon when aligningspinal rods with the anchor heads of implanted screws and hooks duringsurgery.

In one embodiment of the invention, the locking element, which can bedescribed as a collet or collet-style bushing, has an upper portion witha plurality of resilient tabs to initially receive and hold the head ofa pedicle screw. The internal sleeve has a bottom surface with apreferably corresponding inward taper to mate with the tapered shape ofthe exterior surface of the tabs on the collet to allow rotation andfacilitate locking of the collet. The collet has at least one cutout ofpreferably about 50° on its lower side and the anchor head has a lowerportion with a tapered inner surface that together make possible thelarge angulation of the pedicle screw mounted in the collet. The anchorhead preferably also has an internal ledge for receiving a correspondinglip or projection on the collet to seat it within the head and allow itto rotate about the longitudinal axis of the bore of the anchor head sothe cutout can be aligned in a desired direction for full angulation ofthe pedicle screw. The collet may have one or more cutouts andpreferably has multiple cutouts. When the bone anchor is ready to belocked, the bottom interior surface of the internal sleeve presses downon the outside of the tabs of the collet so that the collet compressesaround the screw head to lock the position of the screw.

In another embodiment of the invention, the locking element, which maybe described as a spherical bushing, can rotate or swivel within theanchor head prior to locking. The anchor head has a lower portion with aspherically-cut inner surface that facilitates rotation of die sphericalbushing about a point within the anchor head. The spherical bushing hasa spherical exterior shape, a spherical interior shape, and preferablyat least one slot that permits the bushing to compress the head of apedicle screw or hook inserted into the interior of the sphericalbushing. Preferably, the pedicle screw or hook has an arcuate orspherical upper portion (head) whose shape corresponds to the interiorshape of the spherical bushing. The internal sleeve has a bottominterior surface with a spherical shape to mate with the exteriorspherical shape of the top portion of the spherical bushing. Theinterior surface of the spherical bushing has a centerpoint that ispreferably offset from the centerpoint of the exterior surface of thespherical bushing and hence the pedicle screw mounted within it. Thisoffset provides additional angulation as follows: The pedicle screwangulates a certain amount before its shank engages an edge of diespherical bushing. The spherical bushing can then rotate with thepedicle screw to provide the additional amount of angulation, the sum ofwhich provides the increased angulation. When the bone anchor is readyto be locked, the internal sleeve is pressed down on the sphericalbushing's top surface, so that the bushing compresses around the screwhead to lock the position of the screw.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will be better understood in conjunction withthe accompanying drawings, in which like reference characters refer tolike parts throughout, and in which:

FIGS. 1-3 are perspective, side cross-sectional, and frontcross-sectional views, respectively, of a first embodiment of apolyaxial bone anchor;

FIG. 4 is an exploded view of the polyaxial bone anchor of FIGS. 1-3;

FIGS. 5A and 5B are elevational views of the locking element of thepolyaxial bone anchor of FIGS. 1-4;

FIG. 6 is a perspective view of the locking element of FIGS. 1-4 with apedicle screw mounted therein;

FIG. 7 is a side cross-sectional view of the anchor head of thepolyaxial bone anchor of FIGS. 1-4;

FIG. 8 is a perspective view showing the bottom of the internal sleeveof the polyaxial bone anchor of FIGS. 1-4;

FIGS. 9-11 are perspective, side cross-sectional, and frontcross-sectional views, respectively, of a second embodiment of apolyaxial bone anchor;

FIG. 12 is an exploded view of the polyaxial bone anchor of FIGS. 9-11;

FIG. 13 is a cross-sectional view of the locking element of thepolyaxial bone anchor of FIGS. 9-12;

FIGS. 14 and 15 are perspective views of two embodiments, respectively,of the locking element of the polyaxial bone anchor of FIGS. 9-12;

FIG. 16 is a side cross-sectional view of the anchor head of thepolyaxial bone anchor of FIGS. 9-12; and

FIG. 17 is a perspective view showing the bottom of the internal sleeveof the polyaxial bone anchor of FIGS. 9-12.

DETAILED DESCRIPTION OF THE INVENTION

The invention can be used to treat various spinal disorders including,for example, degenerative instabilities and instabilities due todecompression, tumors, infections, and fractures.

Note that while the polyaxial bone anchor is described and illustratedherein with reference to certain preferred or exemplary embodiments, theinvention should not be limited to those preferred or exemplaryembodiments. Furthermore, the features described and illustrated hereincan be used singularly or in combination with other features andembodiments.

FIGS. 1-3 show a first embodiment of a polyaxial bone anchor. Polyaxialbone anchor 100 includes a fastener 102, an anchor head 104, and ananchor member 106. Fastener 102 is a locking cap that includes a lockingring 112 and a set screw 112 and may be similar or identical to thatdescribed in International Patent Application PCT/US2006/015692,internationally filed Apr. 25, 2006, which is incorporated herein byreference in its entirety. Alternatively, fastener 102 may be any knownfastener, and anchor head 104 may have any corresponding featuresrequired to permit attachment and operation of fastener 102 (e.g.,threaded upper arms). Anchor head 104 is preferably cylindrically hollowhaving a generally longitudinal bore 1014 along longitudinal axis 109.Anchor head 104 also has a generally U-shaped opening 103 transverse tolongitudinal bore 1014 for receiving a spinal rod 108 or other similarpart. Longitudinal bore 1014 has a top opening 194 and a bottom opening184. Anchor member 106, which may be a bone or pedicle screw, hook, orother similar structure (and will be referred to hereinafter as pediclescrew 106), extends out of bottom opening 184. Anchor member 106 may becoupled to anchor head 104 such that the head and screw can polyaxiallyrotate with respect to each other when in an unlocked position, but theangle of the longitudinal axis of anchor member 106 may be fixed withrespect to the longitudinal axis of anchor head 104 in a lockedposition.

One or more polyaxial bone anchors 100 may be attached, for example, tothe vertebrae via respective anchor members 106, and a spinal rod 108 orother similar part can be inserted into the U-shaped openings 103. Thespinal rod may thereafter be locked with respect to anchor head 104. Asystem of bone anchors and rods could be used to correctly align thespine or treat other spinal disorders.

Representative dimensions of bone anchor 100 include an anchor headheight 114 of about 11.5 mm, a width 124 of about 9.5 mm, and a length134 of about 8.2 mm. Pedicle screw 106 has a shank diameter 116 of about4 mm, a neck diameter 126 of about 2.75 mm, and head diameter 136 ofabout 5.4 mm. Alternatively, bone anchor 100 may be of other dimensions.

Advantageously, pedicle screw 106 can angularly rotate (before beinglocked or clamped in place) about central axis 109 by an angle ⊖ ofpreferably about 50° in any direction (i.e., the angular rotation of thehead of anchor member 106 in the anchor head forms a cone of preferablyabout 100°).

FIG. 4 shows an exploded view of the assembly of bone anchor 100, whichincludes locking ring 112 and set screw 122 of locking cap 102, a hollowgenerally cylindrically shaped internal sleeve 405, a spinal rod 108, aninternal locking element 407, a pedicle screw 106 (shown mounted inlocking element 407), and an anchor head 104. Bone anchor 100 is firstassembled by snap-fitting locking element 407 over the head of pediclescrew 106.

As shown in FIGS. 5A and B, locking element 407 may be described as acollet or collet-styled bushing (referred to hereinafter as collet 407).Collet 407 is made of a resilient material that can be compressed aroundthe head of pedicle screw 106 to retain pedicle screw 106 securely inplace. Preferably the material of the collet is softer than the materialof internal sleeve 405 and pedicle screw 106.

Preferably, the upper portion 487 of collet 407 provides the collet withmost, it not all, of its screw head retention capabilities. Upperportion 487 has a plurality of resilient tabs 427. The exterior surfaceof tabs 427 preferably are tapered inward at an angle φ of preferablyabout 30°, although angles φ are alternatively possible andcontemplated. Tabs 427 can deflect outward to allow the head 186 ofpedicle screw 106 to be inserted within al space of the collet, as shownin FIG. 6. The internal space 477 of collet 407 is shaped tosubstantially match the shape of the pedicle screw head such that thecollet has to be pressed over the screw head in a friction fit.Preferably, the head 186 of the pedicle screw and the internal space ofthe collet have at least a portion which is spherically shaped. Tabs 427are separated by slots 437, which may also have a radius or circularshaped portion 447 as a stress relief and/or to provide betterresiliency to tabs 427. The arrangement, shapes, and dimensions of thetabs/slots optionally may be different than shown.

The collet/screw assembly is then inserted screw-shank first through thetop opening 194 in anchor head 104 until circumferential lip 417 ofcollet 407 is seated against circumferential internal ledge 144 ofanchor head 104 (see FIGS. 2, 3, and 7). Screw shank 146 of pediclescrew 106 now protrudes through the bottom opening 184 of anchor head104 as shown in FIGS. 1-3. The diameter of collet 407 at lip 417preferably is such that it can pass through the top opening 194 ofanchor head 104, but cannot pass through the bottom opening 184 and moreparticularly internal ledge 144 of anchor head 104.

The lower portion 497 of collet 407 has one or more cutouts 457 of angleα, which is measured from the bottom of collet 407 at central axis 509(which coincides with central axis 109 when collet 407 is seated inanchor head 104) to the top of the cutout, as shown in FIG. 5A.Preferably angle α is about 50°. (other angles α are alternativelypossible). The embodiment of collet 407 shown in FIG. 5A has three 50°cutouts 457. Embodiments with two, four, or more cutouts are possible.Cutouts 457 make possible the larger angulation between the anchor headand pedicle screw where cutouts are located. Collet 407 may rotate orswivel about axis 109 within anchor head 104, prior to the locking ofthe pedicle screw, to position a cutout in a preferred direction inwhich to provide full (i.e., maximum) angulation. For example, if acutout is not aligned as desired, pedicle screw 106 will press against aprong 467 as a surgeon angulates anchor head 104 in a desired direction.This pressing preferably causes collet 407 to rotate within anchor head104 until cutout 457 is aligned in the desired direction.

The number of cutouts represents a tradeoff between versatility andscrew retention capability. That is, a collet with more cutouts has morepositions at which to provide full angulation and is thus more easilyaligned (i.e., such a collet does not need to be rotated as much to bealigned as a collet with fewer cutouts). However, retention capability(e.g., friction and gripping strength) is in part a function of theamount of surface area in contact with the pedicle screw head. If morearea is in contact with the screw head (e.g., because the collet hasfewer cutouts), more friction to provisionally hold the pedicle screw inplace before locking and more gripping strength to lock the screw inplace is available. If more cutouts are provided, less surface area maycontact the pedicle screw head, and less friction and gripping strengthmay be provided.

Also facilitating the angulation of pedicle screw 106 is a preferablytapered lower portion inner surface 1004 of anchor head 104, as shown inFIG. 7. Pedicle screw 106 will angulate until the neck 156 of the screwbutts against inner surface 1004, as best shown in FIG. 2.

Internal sleeve 405, which may be optional in some embodiments, is nextinserted downward into anchor head 104. Internal sleeve 405 preferablyprovides a U-shaped channel 455 transverse to a longitudinal bore insleeve 405. Internal sleeve 405 preferably has a pair of retention tabs415 a,b on its outer surface that snap into respective slots 154 a,b onopposite walls of anchor head 104 (best seen in FIG. 2). This aligns theU-shaped channels of anchor head 104 and sleeve 405. Slots 154 a,b ofanchor head 104 allow sleeve 405 to move up and down from an unlockedscrew position to a locked screw position, respectively, on top ofcollet 407, while retaining the sleeve within the anchor head. Tabs 415a,b may also keep the U-shaped channel in sleeve 405 aligned with theU-shaped opening in the anchor head. Alternatively, other means ofkeeping U-shaped channel 455 in sleeve 405 aligned with U-shaped opening103 in anchor head 104 may used, such as, for example, protruding tabsalong the boundary of U-shaped channel 455 that project or snap intospace provided by the U-shaped opening in anchor head 104.

With fastener 102 removed from the assembly of the anchor head, internalsleeve, collet, and pedicle screw, the pedicle screw may be attached toa bone. The head of pedicle screw 106 preferably has a recess 166 (asshown in FIG. 3) or slot 166 (as shown in FIG. 4) keyed to receive a hexwrench, torque wrench, or other known driver (through the aforementionedassembly) to implant the pedicle screw by rotating into, for example, avertebra.

Anchor head 104 may now be aligned to receive a rod 108. Rod 108 ispreferably snapped into internal sleeve 405. The distance betweenupright arms 425 a,b of sleeve 405 across the narrowest widths 435 ofthe U-shaped channel is preferably slightly less than the diameter ofrod 108. For example, if rod 108 has a diameter of about 3.5 mm, theaforementioned distance would preferably be about 3.26 mm. In thismanner, the sleeve may provisionally retain the spinal rod but stillpermit the rod to slide in the U-shaped channel or be removed.Alternatively or additionally, sleeve 405, with or without the spinalrod, can be pushed down in the anchor head (e.g., be pushing down on thespinal rod in the U-shaped channel) so that the under surface of sleeve405 interacts with tabs 427 on collet 407 to provisionally lock thepedicle screw with respect to the anchor head. In this manner, thespinal rod is still permitted to slide within and/or be removed from thesleeve.

With the spinal rod in the U-shaped channel (with or without the head ofthe screw or hook being locked in the anchor head), the locking cap 102may be placed on anchor head 104, closing the U-shaped channel. In thisembodiment, locking cap 102 is first positioned on top of anchor head104 and pressed downward until it snaps into position. The locking capis then rotated until oppositely-positioned projections 132 a,b onlocking ring 112 contact corresponding structures 164 a,b, respectively,on anchor head 104. As this occurs, a pair of oppositely-positioned,preferably dovetailed, lateral flanges 142 a,b on locking ring 112 slidewithin corresponding, preferably dovetailed, grooves 174 a,b,respectively, on anchor head 104. Preferably, locking ring 112 and theupper surfaces of anchor head 104 do not engage each other with screwthreads, although screw threads may be used, as well as differentlocking caps.

At this stage, rod 108 can still be positioned (e.g., moved) relative toanchor head 104 and pedicle screw 106. Upon satisfactory positioning ofthe rod and pedicle screw, set screw 122 is driven downward to lock therod and anchor head in place. Set screw 122 has external threads 152that mate with internal threads 162 of locking ring 112. Preferably, theset screw is screwed into the locking ring before the locking cap isinserted into the anchor head, and preferably the set screw cannot bescrewed out of the locking ring because of a flared portion 172 at thebottom of the set screw. Set screw 122 preferably also has a star socket182. Alternatively, set screw 122 can have other types of sockets orrecesses keyed to other known drivers or tools. A single instrument/toolmay be used with locking cap 102 to drive in a single action bothlocking ring 112 and set screw 122 simultaneously to lock locking cap102 in place on the anchor head and then to continue driving set screw122 alone until rod 108 and pedicle screw 106 are clamped in place.

As set screw 122 contacts rod 108, rod 108 pushes down on internalsleeve 405. As the downward rotation of set screw 122 continues, ifinternal sleeve 405 is in the upper position in anchor head 104,internal sleeve 405 moves downward within anchor head 104 compressingand ultimately crush-locking collet 407 around the head of pedicle screw106, locking pedicle screw 106 with respect to anchor head 104. As shownin FIG. 8, internal sleeve has a bottom interior surface 455 preferablytapered inward by preferably about 30° so as to mate with the taperedtabs 427 of collet 407. Set screw 122 may be driven downward until (1)retention tabs 415 a,b of sleeve 405 contact the bottom of slots 154 a,bon anchor head 104, (2) the bottom edge 445 of sleeve 405 and lip 417 ofcollet 407 are clamped against internal ledge 144 of anchor head 104,and/or (3) tabs 427 are compressed against the head of the pedicle screwsuch that the sleeve can no longer travel down the bore of the anchorhead. The set screw will push the spinal rod into the bottom of theU-shaped channel in sleeve 405 in order to move the sleeve down the boreof the anchor head. Once the sleeve can no longer move the sleeve downthe bore of the anchor head, the set screw will apply pressure to thespinal rod so that it becomes locked in a final position in the sleeve(and in anchor head 104) so that the rod cannot slide and/or be removedfrom the anchor head.

Alternatively, other fasteners or caps may be used.

Collet 407 may be advantageously used with other types of anchor heads,internal sleeves, fasteners, and pedicle screws than those shown herein.For example, collet 407 may be used with similar corresponding boneanchor elements disclosed in the previously cited U.S. ProvisionalPatent Application No. 60/674,877, filed Apr. 25, 2005, incorporatedherein by reference in its entirety.

FIGS. 9-11 show a second embodiment of a polyaxial bone anchor.Polyaxial bone anchor 900 includes fastener 102, an anchor head 904, andan anchor member 106. Anchor head 904 is substantially similar to anchorhead 104 and is cylindrically hollow having a longitudinal bore 9014,top opening 9194, bottom opening 9184, and a generally U-shaped opening903 transverse to the longitudinal bore for receiving spinal rod 108 orother similar part. Unlike anchor head 104, however, side lower portion994 of anchor head 904 may have an inward taper. In one embodiment, sidelower portion 994 may taper inward by about 0.65 mm on each side suchthat, for example, an upper width 924 of about 9.5 mm results in a lowerwidth 9004 of about 8.2 mm. Bone anchor 100 may have the taperillustrated in this second embodiment and alternatively, bone anchor 900may not have a taper as illustrated. Other representative dimensions ofbone anchor 900 may be identical to those of bone anchor 100, and boneanchor 900 alternatively may be of other dimensions.

As with bone anchor 100, anchor member 106 (which will again be referredto hereinafter as pedicle screw 106) may be associated with or coupledto anchor head 904 such that the head and screw can polyaxially rotatewith respect to each other. In particular, pedicle screw 106 canadvantageously polyaxially rotate (before being locking or clamped inplace) about central axis 909 of anchor head 904 by an angle ⊖ ofpreferably about 50° in any direction (i.e., the angular rotation of thehead of pedicle screw 106 in anchor head 904 forms a tone of preferablyabout 100°).

FIG. 12 shows an exploded view of the assembly of bone anchor 900, whichincludes locking ring 112 and set screw 122 of locking cap 102, a hollowinternal sleeve 905, spinal rod 108, an internal locking element 907,pedicle screw 106 (shown mounted in locking element 907), and anchorhead 904. The assembly of bone anchor 900 is substantially, if notcompletely, identical to bone anchor 100. Lucking element 907 is firstsnap-fitted onto the head 186 of pedicle screw 106. The lockingelement/screw assembly is then inserted screw-shank first through thetop opening 9194 of anchor head 904 until the lower exterior surface 977of locking element 907 rests against corresponding spherical innersurface 984 on the lower portion of anchor head 904. This causes screwshank 146 of pedicle screw 106 to protrude through the bottom opening9184 of anchor head 904. Internal sleeve 905 may be inserted through topopening 9194 so that the sleeve is retained in anchor head 904.

As shown in FIGS. 12-15, locking element 907 may be described as aspherical bushing (referred to hereinafter as bushing 907). Bushing 907is made of a resilient material that can be compressed around the headof pedicle screw 106 to retain pedicle screw 106 securely in place.Preferably the material of the bushing is softer than the material ofinternal sleeve 905 and pedicle screw 106. Internal space 917 of bushing907 is shaped to substantially match the shape of the pedicle screw headsuch that the bushing has to be pressed over the screw head. Preferably,internal space 917 of the bushing has an arcuate or spherical shape tocorrespond to the preferably spherical or arcuate shape of the head ofthe pedicle screw. The exterior surface 987 of bushing 907 preferablyhas an arcuate or spherical shape. The inner surface 984 of lowerportion 994 of anchor head 904 preferably has a corresponding arcuate orspherical shape so that bushing 907 can rotate or swivel in anchor head904 about a point inside the anchor head and/or within the bore of thespherical bushing.

Bushing 907 has a slot 927 to provide resiliency. Slot 927 may extendcompletely through from the exterior the side of bushing 907 to theinterior side of the bushing and from the top end of the bushing, to thebottom end of the bushing as shown in FIG. 14. Another embodiment of aspherical bushing according to the invention is shown in FIG. 15,Bushing 1507 has additional slots 937 that do not extend completely fromthe top end of the bushing to the bottom end of bushing 1507. Slots 937may also have a radius or circular shaped portion 947 as a stress reliefand/or to provide better resiliency. The arrangement, shapes, anddimensions of the slots of bushings 907/1507 alternatively in bedifferent than shown. For example, although slots 937 are shown asextending from the lower or bottom end of bushing 1507, some or all ofslots 937 alternatively can extend from the top end of bushing 1507.

Bushing 907 (and bushing 1507, referred to collectively hereinafter asbushing 907) can rotate or swivel about a point in the interior of theanchor head and/or bushing within anchor head 904 prior to locking.Inner surface 984 of anchor head 904 facilitates the rotation of bushing907. As shown in FIG. 13, the inner surface of bushing 907 has aspherical centerpoint 957 that is preferably offset from the sphericalcenterpoint 196 of the exterior surface of bushing 907. This offset 967is preferably about 0.6 mm (alternatively, offset 967 can be of otherdimensions). In use, the pedicle screw may first angulate a certainamount until its shank 146 engages a lower edge 977 of bushing 907. Atthat point, bushing 907 can rotate with the pedicle screw to provide anadditional amount of angulation, the sum of which provides the totalangulation of the screw within the anchor head. The angulation of thescrew within the bushing is preferably up to about 20° to about 30° ofmovement and the angulation of the bushing within the anchor head ispreferably up to about 20° to about 30° of movement.

Alternatively, the centerpoints of bushing 907 and the pedicle screwhead can be the same, which may maximize the retention capability ofbushing 907 with respect to the screw head.

As with bone anchor 100, internal sleeve 905, which may be optional, isnext inserted downward into anchor head 904. Internal sleeve 905 ispositioned on top of bushing 907 and its insertion into and movementwithin anchor head 904 is substantially identical to that of internalsleeve 405 and anchor head 104. That is, internal sleeve 905 has a pairof retention tabs 915 a,b on its outer surface that snap into respectiveslots 954 a,b on opposite walls of anchor head 904. This insertionaliens the U-shaped channels of anchor head 904 and sleeve 905. Slots954 a,b of anchor head 904 allow sleeve 905 to move up and down from anunlocked screw position to a locked screw position, respectively. Thesleeve may have the provisional locking features as described for boneanchor 100. Alternatively, other means of keeping U-shaped channel 955of sleeve 905 aligned with U-shaped opening 903 in anchor head 904 mayused, such as, for example, protruding tabs along the boundary ofU-shaped channel 955 that project or snap into space provided by theU-shaped opening in anchor head 904.

With fastener 102 removed, the pedicle screw may be implanted in a bone,such as, for example, a vertebra, and anchor head 904 can thereafter bealigned to receive a spinal rod 108, which is snapped into internalsleeve 905 in a substantially, if not completely, identical manner asthe corresponding parts of bone anchor 100.

Locking cap 102 is next placed on author head 904 and tightened to lockthe rod and screw positions in a manner identical to that describedabove with respect to bone anchor 100. In particular, as set screw 122contacts rod 108, rod 108 pushes down on internal sleeve 905. Thiscauses internal sleeve 905 to move downward, compression locking bushing907 against the head of pedicle screw 106. As shown in FIG. 17, internalsleeve 905 has a bottom surface 975 with a preferably correspondingarcuate or spherical shape that mates with the top portion of bushing907. As with bone anchor 100, set screw 122 may be driven downward untilretention tabs 915 a,b of sleeve 905 contact the bottom of slots 954 a,bon anchor head 904, until the bottom edge 945 of sleeve 905 raid bushing907 are clamped against surface 984 of anchor head 904, or until sleeve905 contacts bushing 907 so that further movement of sleeve 905 is notpossible. Alternatively, other fasteners or caps may be used.

As with collet 407, bushing 907 may be advantageously used with othertypes of anchor heads, internal sleeves, fasteners, and pedicle screwsthan those shown herein. Bushing 907 may be used with similarcorresponding bone anchor elements disclosed in the previously citedInternational Patent Application PCT/US2000/015692, internationallyfiled Apr. 25, 2006, incorporated herein by reference in its entirety.Bushing 907 provides an additional degree of freedom as compared tocollet 407. Bushing 907 not only will rotate about the longitudinal axisextending through the bore of the anchor head, but will rotate about anaxis extending transverse to the longitudinal axis.

The present invention has been described in connection with thepreferred embodiments. These embodiments, however, are merely forexample and the invention is not restricted thereto. It will beunderstood by those skilled in the art that other variations andmodifications can easily be made within the scope of the invention asdefined by the appended claims, thus it is only intended that thepresent invention be limited by the following claims.

1. (canceled)
 2. A polyaxial bone anchor configured to attach a rod to abone, comprising: an anchor head defining a central axis and alongitudinal bore extending along the central axis, the longitudinalbore having a top opening and a bottom opening, the anchor head furtherdefining a generally U-shaped channel transverse to the longitudinalbore and configured to receive the rod; a bushing configured to beretained within the anchor head, the bushing defining a firstcenterpoint and a second centerpoint offset from the first centerpoint;an anchor member having a first portion that is configured to beretained by the bushing, and a second portion that extends through thebottom opening of the anchor head when the first portion is retained bythe bushing, wherein the anchor member is configured to angulate withrespect to the bushing about the first centerpoint until the secondportion of the anchor member abuts the bushing so as to define a firstrange of angulation with respect to the central axis, and the bushingand the anchor member are configured to angulate together about thesecond centerpoint so as to define a second range of angulation withrespect to the central axis, wherein the second range of angulation isabout 20 degrees to about 30 degrees, and a sum of the first and secondranges of angulation is at least 50 degrees.
 3. The polyaxial boneanchor of claim 2, wherein the bushing defines an inner surface and anouter surface, the inner surface defines the first centerpoint, and theouter surface defines the second centerpoint.
 4. The polyaxial boneanchor of claim 3, wherein one or more of the inner and outer surfacesof the bushing is spherical.
 5. The polyaxial bone anchor of claim 4,wherein: the outer surface of the bushing is spherical; and the anchorhead has a lower portion that defines the bottom opening, the lowerportion further defining a spherical inner surface that corresponds tothe outer surface of the bushing.
 6. The polyaxial bone anchor of claim5, wherein the lower portion of the anchor head defines an outer surfacethat tapers toward the bottom opening.
 7. The polyaxial bone anchor ofclaim 6, wherein the bushing defines a top surface and a bottom surfacespaced from one another, and at least a portion of the bottom surface ofthe bushing extends outward from the bottom opening of the anchor headthroughout an entirety of the second range of angulation.
 8. Thepolyaxial bone anchor of claim 7, further comprising a sleeve retainedwithin the anchor head, the sleeve having a top surface and a bottomsurface spaced from the top surface of the sleeve, the bottom surface ofthe sleeve configured to abut the top surface of the bushing, the sleevedefining a second U-shaped channel aligned with the U-shaped channel ofthe anchor head, the second U-shaped channel extending transverse to thelongitudinal bore, the second U-shaped channel configured to receive therod therein.
 9. The polyaxial bone anchor of claim 8, further comprisinga fastener removably mountable to the anchor head to lock the rod in theU-shaped channel and the second U-shaped channel and press the sleeveagainst the bushing to as to compress the bushing against the firstportion of the anchor member in a manner fixing a relative positionbetween the anchor head and the anchor member.
 10. The polyaxial boneanchor of claim 2, wherein the angulation of the anchor member about thefirst central axis forms a cone of about 100 degrees.
 11. The polyaxialbone anchor of claim 2, wherein the first portion of the anchor memberis configured to angulate with respect to the bushing about the firstcenterpoint, and the bushing comprises a material that is softer than amaterial of the first portion of the anchor member.
 12. The polyaxialbone anchor of claim 2, wherein the bushing and the anchor member areconfigured to angulate together about the second centerpoint so as todefine the second range of angulation after the anchor member hasangulated with respect to the bushing about the first centerpoint andthe second portion of the anchor member has abutted the bushing so as todefine the first range of angulation.
 13. A bone fixation system,comprising: a spinal rod; a first polyaxial bone anchor and a secondpolyaxial bone anchor each configured to be 1) anchored to a respectivefirst and second vertebra and 2) coupled to the spinal rod, so as tomaintain a relative position between the first and second vertebra,wherein each of the first and second polyaxial bone anchors comprises:an anchor head defining a central axis and a longitudinal bore extendingalong the central axis, the longitudinal bore having a top opening and abottom opening, the anchor head further defining a generally U-shapedchannel transverse to the longitudinal bore and configured to receivethe spinal rod; a bushing configured to be retained within the anchorhead, the bushing defining a first centerpoint and a second centerpointoffset from the first centerpoint; an anchor member having a firstportion that is configured to be retained by the bushing, and a secondportion that is configured to extend through the bottom opening of theanchor head and into the respective first or second vertebra, whereinthe anchor member is configured to angulate with respect to the bushingabout the first centerpoint so as to define a first range of angulationwith respect to the central axis, and the bushing and the anchor memberare configured to angulate together about the second centerpoint so asto define a second range of angulation with respect to the central axis,wherein a sum of the first and second ranges of angulation is one ofabout 50 degrees and greater than 50 degrees.
 14. The bone fixationsystem of claim 13, wherein the first range of angulation is about 20degrees and about 30 degrees.
 15. The bone fixation system of claim 14,wherein the second range of angulation is about 20 degrees and about 30degrees.
 16. The bone fixation system of claim 13, wherein at least oneof the bushings has an inner spherical surface that defines the firstcenterpoint and an outer spherical surface that defines the secondcenterpoint.
 17. The bone fixation system of claim 16, wherein the atleast one of the bushings defines a top surface and a bottom surface,and the second centerpoint of the at least one of the bushings islocated between the first centerpoint and the bottom surface of the atleast one of the bushings.
 18. The bone fixation system of claim 16,wherein the first and second centerpoints are offset by about 0.6 mm.19. The bone fixation system of claim 17, wherein at least a portion ofthe at least one of the bushings extends outward from the bottom openingof the respective anchor head throughout an entirety of the respectivesecond range of angulation.
 20. The bone fixation system of claim 12,wherein each anchor head comprises a first material and each bushingcomprises a second material that is softer than the first material, andthe second material is a resilient material configured to allow eachbushing to be compressed around the first portion of the respectiveanchor member to retain the respective anchor member in positionrelative to the respective bushing.
 21. The bone fixation system ofclaim 12, wherein each of the first and second polyaxial bone anchorsincludes a fastener removably mountable to the respective anchor head tolock the rod in the respective U-shaped channel and the respectivesecond U-shaped channel and compress the respective bushing against thefirst portion of the respective anchor member in a manner fixing arelative position between the spinal rod and the respective anchormember.